Through the Science of Bioacoustics
If only we could ‘see’ sound, the whole acoustical spectrum of intricate natural ‘noise’ would be laid bare in front of our eyes. We would see then that the acoustical environment is not simply a random and orderless cacophony of animal sounds, but is in fact a palpable ecological dimension of any natural ecosystem.
“[l] have long been fascinated by the ways in which hunters from non-industrial societies determine types, numbers, and conditions of game and other creatures hundreds of meters distant through dark forest undergrowth by sound where nothing appears to the Western [modern] eye or our untrained ear to be especially distinct. As we are primarily a visual culture, no longer connected to what environments can tell us through sound, we’ve lost aural acuity once central to the dynamic of our lives,” wrote Bernie Krause, a soundscape ecologist and musician, in 1993.
Dabbling in acoustics is the exclusive realm of animals. The sound waves travel through mediums such as air and water, and most animals are neurally equipped to be receptive to these waves and vibrations through specialised hearing organs and adaptations. Quite interestingly, before land animals developed fully functional ears, some of the earliest animals including fish and early terrestrial animals, it is speculated, used lungs for hearing! Over millions of years, auditory systems in many terrestrial and aquatic animals got more refined and sophisticated. They developed the ability to produce vocal and mechanical sounds. The natural scene on the planet exploded into a scintillating orchestra of animal chorus. And the door to endless possibilities of acoustic communication swung open.
Just like animal species vie for suitable ecological niches in a marine or terrestrial habitat to survive, so also do they vie for a viable aural niche within the audio bio-spectrum in order to be heard optimally over and above other sounds. Communicating through vocalisations or mechanical sounds is a crucial aspect of survival for most animals. The transmission and reception of sound signals are used to convey everything from assertion of mating rights, defending territories, individual identification, warning proclamations to food preferences!
The bio-spectrum of an acoustical environment in any wild habitat such as a swathe of dense forest, can be broken down into slots or niches. Every bandwidth of sonic frequency within this spectrum, at a given time, in a particular area, will be claimed by a specific species as its own to avoid competition and masking of its own communication signals. Known as ‘acoustic niche hypothesis’, it means that individual species have adjusted the pitch and amplitude of their sound structure in response to other animal vocalisations and natural sounds, to be heard over the din of ambient noise and successfully communicate with their counterparts. It has taken them several thousand years to find their unique bandwidth and develop signature sounds and acoustical behaviours to survive in their habitat and settle into the equilibrium. The entire spectrum of sound in any given natural ecosystem, is the sum of all the calls and sounds of insects, birds, mammals, etc. combined with the abiotic sounds of wind, water, rustling leaves, creaking branches and whatnot. In a mature and old-growth ecosystem, therefore, sound spectrum is neatly partitioned with native species more or less sticking to their specific bandwidths. Studies prove that today, exceptional human interference is causing erratic and rapid changes to the natural soundscape and the established equilibrium, and several animals including birds are having a hard time adjusting to this ever-changing landscape of ambient noise as they are unable to adapt.
So, can we tell the health of a habitat – terrestrial or otherwise, and its biodiversity, by simply listening to them?
The fascinating field of bioacoustics that has re-emerged with renewed vigour in recent times, strives to answer such larger ecological questions. The implications of acoustic research in ecology are proving to be invaluable in developing our understanding of species, ecosystems, and biodiversity in ways and dimensions never explored before. Broadening the focus of attention to the acoustic landscape as a whole to learn about an ecosystem rather than only studying calls of individual species in isolation, promises to elevate the research and conservation efforts globally.
Leaping advancements in sound technology have made a whole lot of difference in the development of disciplines such as bioacoustics and soundscape ecology. Audio recorders are getting more refined, cheaper, and smaller. By simply fastening recorders to a tree in a forest, for example, thousands of hours worth of jungle ‘sound’ recordings can be captured and analysed using cutting-edge softwares and algorithms many of which are easily accessible. The audio spectrograms of such recordings are helping to monitor biodiversity in a forest, gauge absence or presence of species, assess changing responses to human disturbance and global warming, and so much more without having to really see the animals.
With sound recording devices, such as this acoustic recorder, and sound analysing softwares becoming more accessible, portable, affordable and cutting-edge, they are being widely used by researchers to study natural habitats over large areas. Photo credit: Monkachino/Public Domain
Bioacoustic methods could, in the near future, help to gather field data for which scientists have so far relied on the more traditional, laborious and costly methods such as field surveys and camera-trapping exercises. The advantage of audio recorders is that they can listen in on sounds over a larger area than a camera can see. The acoustic recorders could thus allow detection of many rare animals, especially birds, and aid in monitoring of biodiversity surpassing limitations and weaknesses of traditional field surveys.
“We created a detection framework using Automated Recording Units (ARUs), programmed to record between dawn and dusk for a period of five months to try and monitor the cryptic and nocturnal, Critically Endangered Jerdon’s Courser (Rhinoptilus bitorquatus). We used a template of the Jerdon’s Courser’s call previously recorded by Dr. P. Jeganathan to screen our recordings to find the target species,” says Chiti Arvind, PhD student – IISER Tirupati, who is part of a project which involves efforts to detect the critically endangered bird species that was last recorded in 2008 using bioacoustical detection methods.
“Bioacoustics technology is currently gaining momentum in the detection of elusive and cryptic endangered species. Our lab at IISER Tirupati, in collaboration with other researchers, is working with this technique on different endangered birds. This was the first time that it was tried out on a critically endangered species in India. Bioacoustical studies have several advantages in being less labour intensive, they can be programmed to run for long continuous durations without human intervention and the data can be stored for later investigation,” she adds.
Chiti Arvind (left) along with a team member engaged in field deployment of recorders. Photo credit: P. Jeganathan
While we humans can hear some sounds such as birdsongs, gushing streams, stridulating crickets, roaring winds, there are certain sounds whose frequencies are too low or too high for us to register. The normal human hearing range or audible range is between 20 Hertz (Hz) and 20,000 Hz, while we are most sensitive to pitch in the range of 2,000-5,000 Hz. But, some animals, for example various species of echolocating bats, emit sounds of much higher frequencies, well beyond our hearing range. Thus, we cannot hear the ultrasonic sounds that bats produce. But, technology has taken care of that, and bat calls can now be recorded using special devices called ‘bat detectors’ which are nothing but ultrasonic recorders.
Rohit Chakravarty, a PhD student at the Leibniz Institute for Zoo and Wildlife Research, Berlin, explains, “The use of bioacoustics has revolutionised the way we look at bats. A bat detector allows you to identify different bat species from their characteristic echolocation calls. Large-scale public outreach and citizen science activities (with due training) can also be carried out using bioacoustics making it that indispensable bridge between a hobbyist bat-watcher and a professional bat researcher. Apart from species identification, a bat detector gives you insights into the annual cycle of bats. Unlike birds, bats use most of their calls for a completely different purposes i.e. navigation, orientation and foraging. This predisposes the call to variations with respect to environmental conditions.”
But, how does one glean so much information from sounds that one can’t even hear?
“Sound can be visualised using a time-frequency (X-Y) graph called ‘spectrogram’, not sonograms as wrongly termed. A spectrogram allows you to see the structure of a call, its frequency modulation, the duration of each sound pulse and the same between two subsequent pulses. This is what a bat call library is all about – seeing calls and making measurements from a spectrogram,” explains Chakravarty.
The above spectrogram of a greater yellow house bat recording shows what is called a “feeding buzz”. When the bat tries to hunt an insect, it first calls slowly emitting sound out in all directions. Upon locating an insect prey, it increases the speed at which it calls and that creates a buzz. Credit: Rohit Chakravarty https://bit.ly/37iFSST
This spectrogram of a Kashmir Cave Bat recording shows a social call – a call that this bat is using to communicate with its peers (unfortunately, we don’t know the exact context behind the call). But, there’s something else worth appreciating in the spectrogram. Do you see that the call looks ‘dotted’? That’s because this bat hunts over water, so the sound first bounces off the ripples and then reaches the detector! Credit: Rohit Chakravarty https://bit.ly/37iFSST
“Bioacoustics, presently, does have its limitations; a major one being the lack of reference libraries for identifying echolocation calls. Many bat researchers in India, including myself have now worked towards filling that knowledge gap,” adds Chakravarty. He has created a unique reference library of echolocating bat call recordings of about 57 bat species found in India, so far, and has made them available on the public domain in the form of a very interesting blog. You can access it here – https://blahkanas.wordpress.com/indian-bat-call-library/https://blahkanas.wordpress.com/indian-bat-call-library/
The elaborate role of sound in the evolution of animals on Earth cannot be underestimated. By simply listening to nature’s symphony closely, we will be able to broaden our understanding of ecosystems like never before. Bioacoustics is already transforming the world’s approach to ecological studies and conservation.
“It is the acoustical fabric into which that song is woven that offers up an elixir of formidable intelligence that enlightens us about ourselves, our past, and the very creatures we have longed so earnestly to know,” wrote Krause.
About the author: Purva Variyar is a conservationist, science communicator and conservation writer. She works with the Wildlife Conservation Trust and has previously worked with Sanctuary Nature Foundation and The Gerry Martin Project.
Disclaimer: The author is associated with Wildlife Conservation Trust. The views and opinions expressed in the article are her own and do not necessarily reflect the views and opinions of Wildlife Conservation Trust.
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